security/clevis: NEW PORT a pluggable framework for automated decryption

[dch]

Summary

Using security/tang as an example, it is possible for clevis to retrieve
a previously encrypted secret from the stateless tang server, without
the server having any knowledge of the secret.

Plugins, called pins, can be nested and combined in various ways,
from network presence, to Shamir secret sharing, and TPM2 hardware.

WIP - requires changes upstream.

[dch]

ETOOMUCHWORK